Is this battle during a Sandstorm? While I don't remember battling her during one, Cynthia's Garchomp has Sand Veil and it increases its evasion in a sand storm by 20%.

This is the only reason I can think of because moves like Ice Beam should not miss, unless your Pokemon has Hustle or your opponent's used a move like Flash to decrease your accuracy. Articuno's ability is Pressure, and at this stage an opponent should not be using accuracy reducing moves (they might still use evasion inducing ones), so Sand Veil is the only thing raising Garchomps evasion stat.